1. Field of the Invention
The present invention relates to combination transmitting and receiving, capacitance type, electrostatic transducers capable of transmitting and receiving a burst of ultrasonic energy in general, and to the configuration of the grooves in the backplate member of such transducers, in particular.
2. Description of the Prior Art
A capacitance type electrostatic transducer capable of transmitting ultrasonic energy and sensing a reflection or echo of said transmitted energy, is described in U.S. Pat. No. 4,081,626 to MUGGLI, et al. In such a transducer, a thin plastic film, metallized on one surface to form an electrode, is stretched over a relatively massive metallic counter-electrode, hereinafter termed the backplate, with the non-conductive surface of said film in contact with said backplate. The metallized surface of the film separated by the insulating film from the backplate defines a capacitor such that when a dc bias voltage is applied across the electrodes of this capacitor, irregularities on the surface of the backplate set up localized concentrated electric fields in the film. When a signal is superimposed on the dc bias during a transmission mode of operation, the film is stressed and oscillatory formations develop, causing ultrasonic energy or an "acoustical" wavefront to be propagated from the film with its metallized surface, said combination also being referred to herein as a diaphragm. During the receive mode, varying ultrasonic pressure waves on the diaphragm deform the insulating film, thereby producing a variable voltage across said electrodes.
The above-mentioned irregular transducer backplate surface includes a plurality of concentric, circular grooves, regularly spaced from one another, whose dimensions materially affect a transducer's ultrasonic energy transmission pattern. When great numbers of electrostatic transducers are fabricated for use in mass produced systems, such as the ultrasonic ranging system for an automatically focused camera disclosed in U.S. patent application Ser. No. 3,371, filed Jan. 5, 1979, by J. MUGGLI, now U.S. Pat. No. 4,199,246, it is essential that the transmission patterns of such transducers be uniform or consistent so that approximately the same ultrasonic energy transmission pattern will result from a particular transducer drive signal from one transducer to another. A transmission pattern that is, for example, larger or smaller than a desired or expected transmission pattern, may render a system incorporating an ultrasonic transducer with either of such patterns ineffective.
Presently available electrostatic transducer backplates are normally produced by a die forming operation in which a piece of metal usually disc-shaped and softer than that of the die metal, has the above-mentioned concentric grooves pressed into one side thereof. A die lubricant is applied to the die to facilitate movement of the die-formed portion of said disc-shaped piece of metal or workpiece during said die forming operation and for removal of said workpiece after completion of said die forming operation. As an annular or circular groove shape is pressed into the blank metallic disc during a die forming operation, the die lubricant and air become trapped in each circular groove, between the groove forming die tool and said workpiece. This trapped lubricant is relatively incompressible and because of this property, the pressurized lubricant, and to a much lesser extent the trapped pressurized air, cause a non-uniform and unpredictable increase in groove size (primarily groove depth) as the grooves are die formed. This increased groove size can detrimentally effect the size, shape and/or predictability of the ultrasonic energy transmission pattern of an electrostatic transducer that utilizes such a backplate.